libceph: fix crush_choose_firstn() kernel-doc warnings

Currently, when built with "make W=1", the following warnings are
generated:

net/ceph/crush/mapper.c:466: warning: Function parameter or struct member 'work' not described in 'crush_choose_firstn'
net/ceph/crush/mapper.c:466: warning: Function parameter or struct member 'weight' not described in 'crush_choose_firstn'
net/ceph/crush/mapper.c:466: warning: Function parameter or struct member 'weight_max' not described in 'crush_choose_firstn'
net/ceph/crush/mapper.c:466: warning: Function parameter or struct member 'choose_args' not described in 'crush_choose_firstn'

Update the crush_choose_firstn() kernel-doc to document these
parameters.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>